Tuesday, March 13, 2007

Regrouping or What?

SSTs remain warm in a horseshoe pattern centered on 0/150E and extend to just southwest of Hawaii as well as into the South Pacific. Anomalies are ~plus 0.5-1.0C with actual SSTs from 29-30C. Other weak-moderate positive tropical SST anomalies are also present across portions of the Indian Ocean, far eastern Pacific and the North Tropical Atlantic. Negative anomalies of roughly 0.5-1.0C are present from Indonesia into Northern Australia as the result of recent tropical cyclone and heavy precipitation activity. Cool anomalies of ~0.5C also remain on the equator from about 120-150W with colder subsurface values down to 200m at the date line per latest TAO data. The latter may be attributable to a developing La-Nina. If one does develop, we expect the cooling to occur in “steps” related to forcing from subseasonal events.

Resolving the situation with the tropical convective forcing is difficult and about as tough as it gets in terms of uncertainty. A very weak dynamical signal with the recent weak-moderate MJO remains around 90W and appears to be stalling just west of South America. Tropical convection has also tried to get better organized ~0/140-150E for the past several days, largely forced by the warm SSTs in that region and favorable upper level winds. A Rossby wave energy dispersion from the Southern Hemisphere extratropics has led to an increase in upper tropospheric easterly wind anomalies across the west central Pacific. The locally strengthened upper level divergence is providing a source of lift for additional thunderstorm activity.

Animations of 150mb daily mean vector wind anomalies suggest twin subtropical anticyclones redeveloping ~150E. However, whether or not this convection will then shift east toward the warm SSTs as a MJO-like signal is unclear. Currently, a component actually shifting northwest from the west Pacific toward the Philippines is only complicating matters. Furthermore, should a strong eastward shift occur, the bulk of the forcing will stay south of the equator given the cool east Pacific SSTs and the time of year.

The uncertainty in the near term tropical convective forcing is not present in all circulation fields. Global relative atmospheric angular momentum (AAM) remains low at about -1.5 sigma anomaly. A large contribution is coming from the northern subtropical atmosphere centered ~25N. Zonal mean easterly wind anomalies at 200mb are from 5-10m/s in that region, having propagated off the equator starting back in mid February tied to the recent MJO. There have been a couple of episodes of weak positive global AAM tendency since then (~10 Hadleys), partly linked to variations involving mountain and frictional torques in addition to the MJO dynamical signal moving into the Western Hemisphere. For the latter, weak upper tropospheric westerly wind anomalies developed along the Western Hemisphere equator and are interacting with a subtropical wave train currently moving through South Asia.

A very strong storm system is currently pounding Tibet and northern India, part of the wave energy dispersion along the South Asian wave train. As this system interacts with the west central Pacific convection, an extension of the East Asia Jet (EAJ) is probable. Synoptically, this translates to the current trough just east of Japan moving east toward western North America while the upstream system replaces it. This was a possibility discussed in my March 9th posting, and most models predict this situation starting around next Tuesday. However, details remain very unclear. Nevertheless, there is hope for at least one IOP for the American River Basin HMT project with this system. This trough should then move into the Rockies and Plains during week 2 increasing the probability of high impact weather.

Afterwards, for weeks 2-3, uncertainty remains very high. I would think a GSDM Stage 1 base state (consistent with La-Nina) is most probable to persist, with subseasonal variations. Considering the seasonal cycle more strong troughs for particularly the Rockies and Plains while ridge amplification occurs across the central/east Pacific would be expected. However, if the tropical forcing shifts east along the equator to the date line (and/or a strong central Pacific flare-up occurs), the circulation could quickly transition to Stage 3.

Please note: These are probabilistic statements, which we will try to quantify in future posts. I am on TDY at ESRL/PSD with the HMT project until April 3rd. I will try to post another discussion this Friday. The WB (2007) paper on the GSDM has been published in the February issue of MWR.